1. Field of the Invention
This invention relates to a two-component developer used in electrophotography, electrostatic recording and electrostatic printing, and a magnetic carrier included in the developer.
2. Related Background Art
Conventionally, in image forming apparatus making use of electrophotography, such as printers and copying machines, two-component developers containing toners and carriers are preferably used from the viewpoint of image quality, running performance and high-speed adaptability. As a method of performing development using such two-component developers, in order to secure sufficient image density and enhance fine-line reproducibility, a method is employed in which development is performed in the state a magnetic brush of a developer is brought into contact with a photosensitive member, the peripheral speed of a developing sleeve is made higher than the peripheral speed of the photosensitive member, and an alternating electric field and a direct-current electric field are superimposed.
As magnetic carriers used in such a contact two-component development, they are required to have dielectric breakdown strength resistant to applied voltage, and hence magnetic materials such as ferrite and magnetite the particle surfaces of which are coated with insulating resins are used. However, the carrier is made insulative by the resin coating and comes not to function as a development electrode at the time of development, thereby causing image defects such as called blank areas where an edge effect appears between halftone and solid black.
To remedy such image defects and further to stabilize charging of the toner over a long period of time, it has been proposed that fine resin particles containing a conductive powder is dispersed in a coat material containing a coat resin (see, e.g., Japanese Patent Application Laid-Open No. H10-307429, pages 2 to 4). It has been further proposed to use as a coat material a resin having a critical surface tension of 35 dyne/cm or less and in which fine resin particles and a conducting material have been dispersed (see, e.g., Japanese Patent No. 3173374, pages 2 to 6 and Table 1).
The above means can prevent image defects from occurring and further prevent carrier particle surfaces from contamination (spent). However, since ferrite particles are used as core particles, brush images of a developer magnetic brush tend to appear in the contact two-component development, and also the stress applied to the toner when low-consumption printing is continued may cause the toner to deteriorate, resulting in such a problem that it is difficult for the toner to come away from the carrier.
Accordingly, it has been proposed to use a magnetic carrier of a magnetic fine particle dispersion type in order to lower magnetic force and increase electrical resistance (see, e.g., Japanese Patent Application Laid-Open No. H09-281807, pages 2 to 8).
It has been further proposed to prevent toner-spent by the use of a magnetic carrier whose magnetic fine particle dispersed cores have been coated on their surfaces with a aminosilane coupling agent and a resin having a unit such as a fluoroalkyl unit or a methylene unit (see, e.g., Japanese Patent Application Laid-Open No. 2000-39740, pages 9 to 15 and FIG. 1).
These means allow the carrier to have a high specific resistance, prevent image defects such as blank areas by using a carrier having relatively low magnetic force and rotating the developing sleeve and the photosensitive member counter to each other, afford high image density and superior dot reproducibility, and also remedy carrier contamination. However, where the process speed has been made higher as a result of the achievement of further high speed, the developer magnetic brush in a developing zone may have strong frictional force against the photosensitive member to cause the developer to deteriorate. Also, a system in which the development is performed in the state the developing sleeve and the photosensitive member are rotated contrary to each other tends to cause brush images (white lines) due to the friction of the magnetic brush especially in the case of high-speed processing, as compared with a system in which the development is performed in the state the developing sleeve and the photosensitive member are rotated in the forward direction. Also, the amount of developer (image density) and changes in the amount of developer (gamma curve) with respect to gradations of potential tend to be influenced by changes in, e.g., the distance between the developing sleeve and the photosensitive member, the intensity of the alternating electric field and the amount of the developer held on the developing sleeve.